Ceramic compacts can be prepared in various ways and by different processes. Decisive for the choice of the suitable moulding process are the raw products that are available for the material system in question, the size, the required precision and fidelity to detail of the components to be prepared and, last but not least, economic aspects.
In the preparation of naturally small ceramic compacts for dental prosthetics, the requirements in respect of component precision and accuracy of detail are very strict. For the most part, biocompatible ceramics suitable for use in medicine or dentistry are used in a special fine, highly pure form as raw material.
The preparation of dental restorations is traditionally the work of craftsmen. Dentists remove damaged tooth substance and replace it with plastic filling materials (amalgam, composite). In the case of larger defects, the restoration is prepared by a dental technician. This so-called indirect preparation involves impressions and models, and it is generally a few days before the restoration can be inserted in the mouth of the patient. This traditional preparation by craftsmen is time-consuming, expensive and, for the patient, requires several sessions with the dentist. In addition, the quality of the dental technician's work can scarcely be monitored. To reduce costs, in the last few years the work of dental technicians has increasingly been transferred to low-wage countries.
For the last few years there has also been the possibility of the “automated” preparation of dental prosthetics with CAD/CAM systems. After making a digital impression (scanning in the mouth of the patient or on a duplicate model), the dental prosthetic can be milled or ground from solid material on the basis of the scan data with a milling machine. In the case of minor work, this is even directly possible in the dentist's chair. Waiting times for the patient are thereby clearly reduced and quality checks can be taken into account in the software. However, these preparation processes involving the removal of material have the disadvantage that most of the high-performance ceramic used is lost. In addition, until now the machines used have been expensive, noisy and high-maintenance.
Instead of the material- and tool-intensive processes involving the removal of material, so-called “constructive processes” could be used. The terms “rapid prototyping”, “rapid manufacturing”, “solid freeform fabrication” and “generative manufacturing processes” are often used as synonyms for these “constructive” possibilities. The above-named terms cover various generative manufacturing processes in which 3-dimensional models or components are prepared from computer-aided design data (CAD data) (A. Gebhardt, Vision of Rapid Prototyping, Ber. DGK 83:7-12 (2006)). Examples of typical rapid prototyping processes are stereolithography, 3D printing and inkjet modelling. The principle of rapid prototyping is based on the layered construction of a three-dimensional component. Two-dimensional layers (XY plane) are laid on top of one another. Depending on the thickness of the layers, there is a greater or lesser degree of gradation of the component in the direction of construction (Z direction). It is expected that this preparation is substantially more cost-effective to implement than processes involving the removal of material. The main potential for savings lies in the use of less material. In addition, constructive processes allow a parallel manufacture which will bring a significant saving in time and increase in productivity.
Besides stereolithography (U.S. Pat. No. 5,496,682 A; U.S. Pat. No. 6,117,612 A) 3D printing is also a widespread process for the preparation of ceramic compacts. In this process, a binding-agent solution is printed into a powder bed. The binding agents glue the powder particles together and thus form a consolidated two-dimensional structure from powder. After each layer thus prepared, a new layer of loose powder is applied to the 2D structure into which binding agent is again sprayed. Repeating this step many times produces a three-dimensional article constructed from many layers which can easily be exposed by removing the unconsolidated powder. If ceramic powder is used, the binding agent must be burnt out and the ceramic subsequently compacted by a sintering process.
Although a 3D printing process is described as suitable for the preparation of a dental restoration in U.S. Pat. No. 6,322,728 B1 because of the low packing density of the powder bed and the resultant high porosity of the 3D article after the burning out of the binding agent, it is however generally very difficult to obtain a dense sintered compact from this. Usually, according to this process, mould densities of less than 50% of the theoretical density can be achieved after debinding and compacting, and of less than 95% of the theoretical density after dense sintering. These low densities of the desired workpieces result in only an inadequate final strength and thus can hardly be used as dental workpieces.
A further generative manufacturing process which is suitable for the preparation of ceramic compacts is inkjet printing or inkjet modelling, also called multi-jet modelling or printing when there are several printheads. Analogously to the principle of the standard inkjet printer known from everyday office routine, in this case, 3D articles are printed directly, by delivering liquid, also polymerizable modelling materials (“inks”) in defined drops through one or more nozzles, the ink curing and thus forming the layers in the XY plane. At the same time, an easily removable supporting material can be imprinted. Repeated printing of layers of modelling and optionally supporting material on top of each other produces a three-dimensional article. After separating the printed article from the supporting structure, for example by selective chemical dissolution of the supporting material, a 3D component is left.
The inks used must be very highly liquid (low-viscosity) at the temperature in the pressure nozzle. Furthermore, suitable inks should not contain particles if at all possible, since otherwise the nozzles which typically have a diameter of approximately 100 μm or less can clog up and a continuation of the printing process is prevented. In the case of filler-containing inks, i.e. suspensions of particles in a liquid, only suspensions with a very low fill level have previously been able to be used, wherein the particles themselves must be very small, normally <1 μm, but at least substantially smaller than the average diameter of the nozzle (the printhead manufacturers quote 1/20th of the average nozzle diameter as a guideline). However, the use of fine particles as filler has the consequence that the viscosity of the ink increases and an imprinting is made difficult or prevented because of this.
Until now, suspensions (slips) that contain only a few % by weight of filler have been used as ceramic-filled inks for the inkjet printing of 2D layers or 3D articles. In simple cases, these are suspensions based on water or low-boiling alcohols, during the use of which, however, very thin layers of low height can be produced, but often no actual three-dimensional solids. In addition, defects or cracks often occur as a result of the drying of the printed structures.
Often, therefore, waxes which are imprinted in the hot-melt inkjet printing process are also used to print 3D articles. The waxes are very highly liquid in the printhead at the increased temperatures and solidify when or immediately after striking the surface to be printed on. Where waxes are used as dispersant, higher levels of filling of about 20-30% by vol. can also be achieved, wherein (oxide) ceramic powders can also be used as fillers. An attempt is made to compensate for the high viscosity of the filled waxes that is the inevitable consequence of the high level of filling, by printing at comparatively very high temperatures of 140° C. and above. On the one hand, the printheads and their components such as supply line and nozzle are exposed to a high stress, on the other hand, it is very difficult, if not impossible, to obtain dense, solid ceramic components from the printed 3D articles, even at the named, comparatively high levels of filling of ceramic powder. In addition, the subsequent burning out of the organic constituents is in most cases accompanied by a significant deformation of the printed components and, moreover, the resultant porosity of the debound compacts is too high, i.e. the ceramic density is too low to prepare a dense microstructure by a following sintering process.
DE 10 2006 015 014 discloses a process for the preparation of ceramic moulds by layered inkjet printing of a suspension which contains 50 to 80% by weight ceramic particles, an aqueous boehmite sol, a low-molecular alcohol, a drying inhibitor and an organic deflocculator, followed by drying and curing (sintering) of the layered composite. Preferably, each individual layer is dried before the application of the next layer and after construction of the three-dimensional solid the latter is dried again as a whole. The process is said to be suited to the preparation of tooth implants, inlays, crowns and bridges. The extremely long time required to carry out this process is a disadvantage, since each individual imprinted layer must be dried. Furthermore, the theoretical densities of the sintered moulds that are supposedly achieved with the process correspond to only modest mechanical strength properties in up to 98% of cases.
Seerden et al. (J. Am. Ceram. Soc. 84(11):2514-2520 (2001)) state that unburned ceramic articles that are dimensionally accurate to within <100 μm can be obtained by means of hot-melt inkjet printing of Al2O3 wax slips with a level of filling of up to 30% by vol. Neither the behaviour of the thus-produced 3D articles during debinding and sintering nor the properties of the sintered ceramic compacts are described. The preparation of dental restoration materials is not disclosed.